Concentrations of Credit Risk - Additional Information (Details) customer in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2019 USD ($) customer State subsegment counterparty
Concentration Risk [Line Items]
Number of groups with potential to affect overall exposure | subsegment	3
Number of large counterparties for which credit risk is disclosed | counterparty	5
Number of counterparties with investment-grade rating | counterparty	4
Energy Producers And Their Affiliates
Concentration Risk [Line Items]
Accounts receivable	$ 11.7
Net receivable amount	4.7
Utility Companies And Their Affiliates
Concentration Risk [Line Items]
Accounts receivable	85.0
Net receivable amount	75.5
Five Largest Counterparties
Concentration Risk [Line Items]
Accounts receivable	37.7
Net receivable amount	$ 35.5
Gas Utility
Concentration Risk [Line Items]
Number of customer serves | customer	1.7
Number of state serves across multiple rate classes | State	3